Cancer is a complex and dynamic disease, and many different ways of analysing and classifying tumours have been developed with the aims of determining the degree of tumour progression or invasiveness and the prognosis for the patient, and informing treatment decisions.
Methods of analysing tumours include the assessment of cell morphology in tumours (typically performed by pathologists), measurement of gene expression in tumours (e.g. by microarray analysis), determination of gene mutation status in tumour cells, and evaluating protein expression within tumours (e.g. by immunohistochemical assessment of tumour sections). These methods of analysing tumours are important not only for predicting clinical outcome, but also for informing decisions on patient therapy.
More recently, it has become apparent that the immunological status of tumours can yield useful prognostic information. Accumulating evidence supports the clinical significance of immune response in many cancer types (Galon et al. 2006, Denkert et al. 2010, Loi et al.). Consistent studies have reported associations between immune activity and disease outcome as well as treatment response (Galon et al. 2006, Denkert et al. 2010, Loi et al., Liu et al., Lee et al., DeNardo et al.).
Furthermore, increasing evidence from clinical trials supports the potential of therapies that target immune activity in certain types of cancer (Robert et al., Stagg et al.). This is perhaps best exemplified in late stage melanoma where recent clinical trials have shown an increased survival advantage in patients receiving the monoclonal antibody ipilimumab, which targets the CTLA4 protein receptor that is expressed on the surface of T cells (Robert et al.). This has led to the development of more standardised methods of characterising tumour immune infiltrate in cancers such as the “immunescore” that aims to quantify the in situ immune infiltrate in addition to standardised clinical parameters to aid prognostication and patient selection for immunotherapy in colorectal cancers (Galon et al. 2014).
However, to facilitate the standardisation and reproducibility of scoring immune infiltration, objective approaches are urgently needed (Galon et al. 2014). Furthermore, such approaches need to account for the complexity of immune infiltration into tumours. Abundance, spatial heterogeneity and type of immune cells are the key parameters of immune infiltration (Galon et al. 2014, Fridman et al.). For example, the spatial locations of immune cells have been shown to be useful in predicting the prognosis of colorectal cancer (Galon et al. 2006). Indeed the pathological “immunescore” is based on the numeration of two lymphocyte populations (CD8+ and CD45RO+ cells), both in the core of the tumour and in the invasive margin that maximises the prognostic power (Galon et al. 2014).
Similarly, large-scale studies of breast cancer have demonstrated that pathological assessment of tumour-infiltration lymphocytes based on Hematoxylin & Eosin (H&E) stained core biopsies is a significant predictor for response to neoadjuvant chemotherapy in 1,058 breast cancer samples (Denkert et al. 2010). Recently, a prospective study demonstrated that in HER2-negative breast cancer stromal lymphocytes can be an independent predictor of response to neoadjuvant chemotherapy (Issa-Nummer et al.). Thus, the spatial organisation of lymphocytic infiltration in the context of nearby cancer cells is an important clinicopathological feature of tumours.
In triple-negative breast cancer (TNBC) an active immune response has been associated with favourable prognosis (Loi et al., Liu et al., Denkert et al.). A large-scale immunohistochemistry study of 3,400 breast cancer samples has showed that TNBC is the only subtype of breast cancer to demonstrate a significant link between CD8-positive immune cells and a good prognosis (Liu et al.). Assessment of lymphocytic infiltration based on whole-tumour H&E sections has been associated with favourable outcome in 256 patients after anthracycline-based chemotherapy (Loi et al.). A recent prospective study showed that the presence of tumour-infiltrating lymphocytes in residual tumours after neoadjuvant chemotherapy is predictive of good prognosis in TNBC (Dieci et al.). Given the current lack of targeted molecular treatment and poor clinical outcome of TNBC, this may suggest new therapeutic opportunities for this aggressive tumour type (Stagg et al.). For instance, accumulating data suggest that anthracyclines mediate their action through activation of CD8+ T-cell responses, hence combination with certain immunotherapies could be especially effective for TNBC (Stagg et al.).
However, despite these advances in understanding of the importance of immune infiltration in cancer, there is a lack of reproducible approaches to objectively assess immune infiltration based on pathological sections.